Large concrete shells are being utilized increasingly as roof structures for various types of buildings. Most frequently, these are storage buildings incorporating concrete shells of dome-like configuration. The same type of construction has been used in other buildings, such as churches, requiring high interiors preferably free of pillars or other roof support structures.
The most successful technique for erection of a concrete shell utilizes a single membrane form much like an enormous tent. The form membrane is positioned over the base wall of a building and then inflated. Reinforcing steel mesh is deployed on the membrane form and concrete is then applied over the membrane surface. The membrane form is removed when the concrete is set, and may be employed again for another building.
Another known technique employs a two-layer inflatable membrane form mounted on a frame. The frame is aligned between two building base walls, the membrane form is inflated, reinforcing steel is positioned on the form, and concrete is then deposited on the form surface. After the concrete sets, the form is deflated and the frame assembly is moved to a new position to form a further segment of the total shell. This segmental construction technique is described in Harrington U.S. Pat. No. 3,619,432.
In the use of inflatable membrane forms for the erection of concrete shells, a difficult continuing problem lies in the necessity for securing and sealing the edges of the form membrane to a building base wall or to a form support structure positioned immediately adjacent the base of the building. If the edges of the form membrane are anchored by retaining strips that use nails, bolts, or other like fasteners, tearing of the membrane is a frequent problem, particularly if the membrane form is used repeatedly in constructing a multi-segment shell or in the construction of a plurality of individual shells. Moreover, such conventional fasteners make it difficult to remove the form membrane for subsequent reuse without damage to the membrane.
One example of a technique that avoids the use of conventional fasteners in anchoring an inflatable membrane form for construction of a concrete shell is Neff U.S. Pat. No. 2,388,701. In the Neff patent, the concrete base of a building is poured with an outwardly projecting external ledge at its top. The membrane form extends down into the resulting groove around the periphery of the building and is anchored by bands encircling the entire building. While this arrangement avoids the use of nails, screws, or similar fasteners, it has a distinct disadvantage in that it is limited to buildings with exterior walls of circular or other smoothly curved configuration because any corner on the wall almost inevitably leads to tearing of the membrane form when the anchoring bands are tightened, with potentially disastrous results. The Neff anchoring arrangement is impractical and unusable in buildings of even moderate size because it is virtually impossible to align the membrane form accurately with the groove around the building and to tighten the retaining bands evenly around the complete building. In addition, with this arrangement it is difficult to remove the inflatable membrane form for reuse and removal of the form leaves an open channel around the building periphery.
A clamp assembly used successfully for a number of years in anchoring the edge portion of an inflatable membrane form in construction of a concrete shell is described in Harrington U.S. Pat. No. 3,719,341. That clamp assembly comprises a metal channel mounted on and facing outwardly of a support at the base of the shell; the support is usually the base wall for the building. A wood rail is wrapped in the edge portion of the form membrane and then inserted into the longitudinal opening of the channel. In the embodiment most used commercially, an auxiliary wood rail is also positioned within the channel. That clamp assembly is useful and effective because it requires no nails, screws, or like fasteners. The tension on the membrane produced when the form is inflated helps to hold the rails and the membrane edge in the clamp channel. When the form is deflated, on the other hand, the form membrane can be released from the clamp channel for reuse.
The clamp assembly of Harrington U.S. Pat. No. 3,719,341, however, has some disadvantages. When the form membrane is being secured to the clamp assemblies, close cooperative effort is required between two workmen, one working on the inside of the inflatable form and the other on the outside. If these two workmen are not effective in coordinating their efforts, substantial time can be lost, and the edge portion of the form membrane may be damaged. The form membrane, when inflated, extends back over the top of the clamp channel at an acute angle; if inflation produces tension in a direction outwardly of the longitudinal opening of the channel, the edge portion of the form may be pulled out of the channel. The clamp assembly also presents some difficulty when the concrete shell has set and removal of the form membrane is desired because the concrete at the rim of the shell usually covers the channel opening, making it necessary to remove the entire clamp assembly before the edge of the membrane can be disengaged from the clamp.
Yet another releasable clamp assembly for an inflatable membrane form is described in Harrington U.S. patent application Ser. No. 361,521 filed Mar. 24, 1985 and corresponding to British Patent No. 2,118,608B. That clamp assembly comprises a steel channel mounted at the base of the building, with the channel opening facing upwardly; a guide lip on one leg of the channel projects a short distance over the channel opening. A first rectangular wood rail wrapped in an edge portion of the membrane form is inserted into the channel in a loose fit; a second rectangular wood rail inserted into the top of the channel tightly fills the channel opening immediately below the guide lip and releasably clamps both rails and the edge portion of the membrane form into the channel.
This anchoring arrangement for the inflatable membrane form is substantially better than prior techniques, but there are still substantial problems. The steel channels and the steel mounting members required to secure those channels to the building base are expensive, heavy, and awkward to store. Insertion of the two wood rails into the steel channel is a difficult task, one which the workmen dislike. Furthermore, due to the usual irregularities in the top surface of a building base wall, this form anchoring arrangement is difficult to seal against air leakage when the form is inflated. In addition, when used with large inflatable membrane forms that apply heavy stresses to the clamp steel, there is a tendency for the steel to pull out the bolts employed to mount it on the base wall, particularly since it is usually undesirable to wait for complete curing of the concrete in the base wall prior to erection of the shell. These latter two difficulties apply equally to the clamp arrangement of the earlier Harrington Pat. No. 3,719,432.